Solid-state temperature sensors are widely used to provide reliable temperature measurement for many applications. In fact, silicon sensors often provide superior performance at a much lower cost than resistance-temperature-detectors, thermocouples and thermistors. Silicon-based temperature measurement typically involves an integrated circuit (hereinafter "IC") and a sensor diode as shown in FIG. 1. The IC 100 in FIG. 1 applies current to diode 150 using on-chip current sources 110 and 115 of standard design and corresponding switches S1 and S2 (120 and 125, respectively). Clock and switch control circuit 140 provides alternating clock control signals to alternately close switches S1 and S2 and apply the respective currents to diode 150. Measurement circuit 130 then measures the voltages, V.sub.BE, appearing across the diode when the currents are applied, which voltage is proportional to temperature. Specifically, switch control circuit 140 is shown generating the alternating sampling signals, .PHI..sub.1 and .PHI..sub.2. During clock phase 1, .PHI..sub.1, switch S1 is closed and the diode is biased by I.sub.1 to produce a voltage, V.sub.BE1. During clock phase 2, .PHI.2, switch S2 is closed and the diode is biased by I.sub.2 to produce voltage, V.sub.BE2. The measurement circuit samples and stores the diode voltage during each clock phase.
The measurements are based on a diode's voltage-current relationship, which is governed by the equation EQU V.sub.BE =V.sub.T *ln(I.sub.D /I.sub.S) (1)
where, I.sub.D is the forward diode current, I.sub.S is the diode reverse-saturation current, V.sub.BE is the forward diode voltage, and V.sub.T is the diode's thermal voltage given by EQU V.sub.T =K*T/q (2)
where, K=Boltzmann's Constant=1.38066*10.sup.-23 J/.degree.K
T=Temperature in degrees Kelvin, .degree.K, and
q=Electron Charge=1.602*10.sup.-19 Coulombs.
It can be shown that the change in voltage measured across a diode, .DELTA.V.sub.BE, when the diode is excited with two different currents, I.sub.1 and I.sub.2, is EQU .DELTA.V.sub.BE =V.sub.T *ln(I.sub.1 /I.sub.2) (3)
If the currents, I.sub.1 and I.sub.2, are precisely matched, then .DELTA.V.sub.BE can be used to provide a very stable, well-defined thermometer signal by substituting Equation (2) in Equation (3) and solving Equation (3) for T.
FIG. 1 illustrates a "single-wire" diode temperature measurement system in which only one conducting path connects a single pin of IC 100 and diode 150. The structure of FIG. 1 can, of course, be expanded for use with a plurality N of diodes, each connected to a respective pin on an IC such as 100 in FIG. 1, and each monitoring temperature at a respective off-chip location. A standard N:1 multiplexer is then controlled by the clock and control circuit 140 to connect the measurement circuitry 130 to each of the N diodes in turn.
By way of contrast, FIG. 2 illustrates the use of a transistor 250 to provide a more accurate "two-wire" differential measurement. The IC 200 in FIG. 2 contains an excitation circuit comprising current sources I.sub.1 (215), I.sub.2 (210), and respective switches S1 (220), and S2 (225), operating under the control of clock and switch control circuit 240, as in FIG. 1. Again, the voltages V.sub.BE1 and V.sub.BE2 are sampled and stored when respective currents I.sub.1 and I.sub.2 are applied. In the circuit of FIG. 2 current flows from the excitation circuit output through pin 245 of the IC 200 and through a wire connection 260 having resistance R.sub.WIRE to the emitter of transistor 250. Voltage measurements are made by measurement circuit 240 by way of pin 246 on IC 200. Pin 246, in turn, connects to the emitter of transistor 250 via a separate wire connection 265. Because the current over connection 265 is very low, the IR drop over that wire is likewise low. The use of a separate connection to the emitter 250 avoids possible errors resulting from an IR drop due to the relatively higher applied currents through the sometimes non-negligible resistance of wire 260. In cases where the value for R.sub.WIRE is low, however, the wire 265 may be eliminated and a jumper provided across pins 245 and 246 of IC 200. Because connections 247 and wire 265 are options, they are indicated by dashed lines in FIG. 2. The two-wire connection using transistor 250 (which may be a PNP or NPN transistor) to IC 200 also includes the ground sense path 270 connecting the base of transistor 250 to ground pin at IC 200. Because of the transistor connection, the current flowing in the ground sense wire is divided by the .beta. of transistor 250. Multiplexing techniques can be applied to the circuitry of FIG. 2 in the same manner as for the diode monitor arrangement of FIG. 1. Either PNP or NPN sensor transistors can be used in such applications.
If the currents I.sub.1 and I.sub.2 in the circuits of FIGS. 1 and 2 are precisely in the proportion I.sub.1 =M*I.sub.2, and the measurement circuit is designed to subtract V.sub.BE2 from V.sub.BE1, then the residual signal, .DELTA.V.sub.BE =V.sub.T *ln(M), provides an accurate measure of temperature. A key to achieving accurate temperature measurement using this technique therefore is the matching of the current sources, I.sub.1 and I.sub.2. It is generally not possible to exactly match two independent current sources on a chip with any high degree of reproducibility.
Ratio matching in the range of 0.1% to 1.0% can be achieved through the use of very precise design and manufacturing techniques, but many applications require better accuracy. One-time factory calibration is often used to reduce the current source mismatch error, but this approach relies on the use of expensive manufacturing techniques including thin-film laser trimming, so-called zener-zapping, or fuse blowing. These solutions require additional active silicon for implementation, which increases the size and cost of the chip. Furthermore, the extra trim circuitry and testing results in a reduction of product yield and increase in manufacturing cost.